Broadband communications in the wireless environment may present certain challenges, such as, for example, varying channel conditions and fading, which may limit the data rate achievable on a wireless channel. A highly mobile system, such as, for example, a fast moving train may present further challenges, including, for example, fast fading channel conditions.
Attempts to counter the adverse wireless channel conditions may include transmission power control measures. In particular, it is believed that adapting the transmitter power to channel variations may help achieve increased data rates. In this regard, power control measures for a single-user flat-fading channel, have been discussed, for example, in Goldsmith and Varaiya, “Capacity of fading channels with channel side information,” IEEE Transactions on Information Theory, pp. 1986-1992, November 1997, and Chua and Goldsmith, “Variable-rate variable power MQAM for fading channels,” IEEE Vehicular Technology Conference, 1996, both of which are incorporated by reference as to such measures and their implementation.
Another challenge besetting wireless support for running broadband applications is the severe impact that varying channel conditions may have on data communication protocols adapted to reliable wireless conditions, including, for example, the widely deployed transport layer protocol TCP (Transmission Control Protocol), which is useful for operation over reliable media. In particular, invocation of congestion control mechanisms due to wireless channel errors may adversely affect TCP performance. Various alleviation measures have been proposed and appraised in the literature to handle the problems that TCP faces in a wireless scenario. For example, the proposals include hiding any non-congestion losses from the sender or making the sender aware and capable of differentiating between packet loss due to channel errors and those due to congestion. Several proposals along these lines have been discussed, for example, in Bakakrishman et al., “A comparison of Mechanisms for Improving TCP Performance over Wireless Links,” IEEE/ACM Transactions on Networking, 1998, Xylomenous and Poluzos, “TCP and UDP Performance over a Wireless Lan,” IEEE INFOCOM, 1999, and Arauz et al., “MAITE: A Scheme for Improving the Performance of TCP over Wireless Channels,” IEEE Vehicular Technology Conference, 2001.
Certain network operators may wish to deploy broadband wireless access in a high-mobility environment, such as within high-speed trains in Germany, for example. Delivering wireless computing and communication services, such as, for example, broadband Internet access, to the passengers of high-speed trains may be a challenging task. The ICE trains in Germany, for example, may reach speeds of up to 300 km/h and may pass through tunnels or along rather tall trees. Transmission techniques, such as, for example, satellite links, which require line of sight, may not be feasible for such an endeavor. In particular, the presence of obstacles along the railway routes, such as trees, buildings, bridges, and tunnels, may cause shadowing and/or fading of the satellite link. Moreover, any prospective technology to provide broadband wireless access to high-speed trains may need to serve potentially hundreds of trains concurrently all over Germany. All relevant railroad tracks may be electrified, energy being picked up by two engines at the front and rear of the trains from a catenary wire above the tracks. It is believed that several alternatives like wireless LANs, GPRS, or a combination thereof, are being considered or surveyed to provide broadband wireless access in such a scenario. If TCP is used to carry most or at least a significant portion of the Internet traffic, appropriate tuning of the protocol to the relevant wireless channel conditions may be desired.
Attempts have been made to model TCP performance with power adaptation. A stochastic model of TCP for a generic stationary loss process is discussed, for example, in Altman et al., “A Stochastic Model of TCP/IP with Stationary Random Losses,” ACM SIGCOMM, pp. 231-242, 2000. In the wireless scenario, however, the loss process statistics required by the model may not be easy to model and evaluate, especially due to multipath fading. A TCP throughput expression for wideband CDMA is presented, for example, in Zorzi et al., “Throughput and Energy Performance of TCP on a Wideband CDMA Air Interface,” Wireless Communications and Mobile Computing, Volume 2(1), February 2002. However, this throughput prediction is not based on TCP dynamics modeling but is heuristics-based and hence illustrates TCP behavior in the considered environment.